In data networking systems and computer systems, information signals can be conveyed by light, particularly over long distances. In general, an information-bearing light signal may be generated by one of two approaches. In the first approach, the light signal is generated directly from a light-emitting device whose electrical input is varied to modulate and the light output of the device in relation to the information signal. This generally involves varying the current passing through a semiconductor laser or LED. In the second approach, the output of the constant light source is run through an optical modulator, which varies the amount of light transmitted through itself. The first approach has constraints in terms of output power, modulation speed, extinction ratio, nonlinear-modulation effects, and wavelength chirp. These constraints, in turn, impact several characteristics such as spacing between amplifiers, transmission capacity, transmission distance and receiver performance, distorted signals, and dispersion. All these become more and more significant at data rates above 1 Gbit/sec. Most of these problems are solved by using the second approach (optical modulator), particularly for high-performance systems. However, optical modulation requires a change in the transparency of the modulator material, and only a few physical processes can be fast enough and complete enough to be useful.
Fiber-optic systems and next-generation optical computing systems are extremely demanding for high performance modulation because they require modulators that can switch on and off as often as billions of times in a second, as well as respond very accurately to changes in the input signal. Although a number of technologies have been developed for modulating laser beams, only a few are in practical use, and still fewer meet the strict requirements for the systems. For example, liquid crystals do modulate light, but they are not fast enough for data networking and computing systems.
Electro-optic modulation generally relies upon the electro-optic effect, which is a change in the refractive index of a material in response to the application of an electric field to the material. The change in refractive index affects light passing through the material virtually instantaneously. Increasing the refractive index slows down the light, while decreasing the refractive index speeds up the light. The change is, in general, proportional to the voltage applied to the material.
In a Mach-Zehnder modulator, an input single-mode waveguide is split into two separate waveguide branches (typically of equal length), and then recombined into an exit waveguide after traveling some distance. Since the propagation distance in the modulator branches is short, the light beams in the two separate branches are coherent because they originate from the same source. An electric field is applied to one branch to change the refractive index in the branch (which either decreases or increases the speed of light depending on the direction of electric field), and to thereby create a difference in the propagation speeds of light in the two branches. This in turn creates a phase difference in the light beams. When the phase difference is near or at 180 degrees (one-half wavelength), the two beams will interfere with each other when combining at the exit waveguide, and little to no light will be propagated through the modulator. When the phase difference is at or near zero degrees, the two light beams constructively combine with one another at the exit waveguide to allow the light to propagate through the modulator essentially unchanged. The electric field may be varied between two extreme values, thereby giving the optical modulator a digital on-off transmission characteristic. The electric field may also be varied through a continuum of values to cause the output light intensity to vary in a corresponding continuum of values, which would be suitable for analog modulation applications.
In many electro-optic modulators, electric fields are applied across both waveguides, with the same polarity if the two arms have opposite dipole orientations or with the opposite polarities if the two arms have the same dipole orientation. This approach causes an increase in speed of one light beam and a decrease in speed of the other light beam, and it provides the same modulation but with lower operating electric fields, and correspondingly lower operating voltages. Typically two voltage signals are applied to each branch in a superimposed manner, a bias voltage that sets the operating level and a modulation signal voltage conveys the signal content. For example, the bias voltage may set the modulator to normally transmit half of the power of the input light beam to the output, while the modulation signal voltage varies to incrementally change the transmitted power above and below that midpoint level.
Unfortunately, the electro-optical coefficients of common electro-optic materials are relatively low, which requires the modulators to use relatively long waveguide branches in order to generate a sufficient phase changes with the relatively low electro-optic coefficients. The long waveguide branches take up considerable space on the substrate, and limit the density at which the modulators can be formed on the substrate. Accordingly, there is a need to increase the density of modulators on substrates.